travis: Add srecord package

At least MIPS Boston currently uses srec_cat tool to fiddle with
srecords. There will be other platforms coming, so install the
tool to prevent build problems.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Tom Rini <trini@konsulko.com>